Field of the Invention
The present invention relates to electrical protection circuits for use in multiple phase, alternating current power systems. In commercial and industrial power use, the well-known three phase system is widely employed. However, in many cases, the load equipment coupled to such multiple phase systems comprises single phase elements which are connected between two phase lines, or between one phase line and ground. This type of electrical equipment, as well as three phase equipment, is susceptible to damage caused by a number of undesirable operating conditions. The undesirable conditions are:
A. Phase Unbalance
Unbalance of a three phase power system occurs when single phase loads are applied, causing one or two of the lines to carry more or less of the load. Normally, on new installations of three phase power systems, careful attention is given to balancing the load. However, as more and more single phase loads are added to the system, an unbalance may begin to occur. This phase voltage unbalance can cause three phase motors to run at temperatures beyond their published ratings and, if undetected, can result in serious damage to equipment and machinery. In addition to internal overheating, these excessive temperatures cause insulation breakdown and reduce motor life. Standard thermal overload magnetic breakers, and other such apparatus presently available, generally do not detect the gradual load unbalance and for this reason are not considered adequate protection.
B. Phase Loss/Under Voltage
The total loss or severe under-voltage of one of the three phases is a type of phase unbalance. This condition is generally known as "single phasing." The most serious result of this condition is that it can go undetected on most systems for an extended period of time, thus causing a motor coupled to the lines to burn out, since a three phase motor running on less than three phase power will continue to run by drawing all of its current from the remaining two lines. In most cases, this condition will not be detected by measuring the voltage at the motor terminals because the open winding of the motor is generating a voltage nearly equal to the phase voltage that was lost.
Phase loss can also occur when a single phase overload condition causes a fuse to blow, by a three phase circuit being struck by lightning, or by a mechanical failure within the switching equipment associated with the machinery. Attempting to start a three phase motor on single phase power will cause the motor to draw locked-rotor current. Thermal overloads placed in the lines are not always capable of preventing damage to the motor under these conditions. Loss of power on any one phase can also cause all single phase equipment connected to this phase to be operating at extremely low voltage. This could result in equipment damage.
C. Phase Reversal
The reversing of any two of the three phases can cause damage to electrically powered machinery or may result in injury to personnel. This can occur when modifications are made to power distribution systems or when maintenance is performed on cabling and switching equipment. The National Electrical Code requires phase reversal protection on all equipment transporting people, such as elevators and escalators.
D. Circuit Over/Under Voltage
When the voltage existing on all of the three phases exceeds a predetermined limit, the loads coupled to those three phase lines may be damaged by an electrical breakdown in the wiring insulation, or by a thermal overload caused by excessive power output from the load. Such over-voltages can be caused by lightning strikes somewhere along the power distribution system or by momentary shorts within the primary windings of transformers along the power distribution system.
Under-voltages on all phases of the multi-phase line can be caused by a defective generator or by excessive loads being placed upon the power distribution system which exceed the capacity of the generator. Such under-voltage situations occur after a momentary power loss when many electrical motors, each drawing a very large starting current, are simultaneously started by the reapplication of power to the three phase lines. When an under-voltage situation occurs on all phases of the power distribution system, some loads, such as certain types of motors, tend to draw excessive current in order to maintain a specified power output. This excessive current drawn by the motors causes a rapid heat build up within the motor, which in turn can cause failure of the insulation or an overheating of critical parts.